Grease and flour a 9”x13” baking pan. Line bottom of pan with parchment paper; set aside.
Preheat oven to 350°F.
Using a medium bowl, sift together the flour, baking powder, and salt. Stir until well combined; set aside.
Using a countertop mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy (~2-3 minutes on medium speed). Add the egg and egg whites; mix on low speed until well combined.
Add half of the flour mixture to the bowl; mix on low speed until well combined.
Add the milk, sour cream, vanilla extract and lemon juice (optional); mix on low speed until well combined. Add the remaining flour mixture; mix on low speed until well combined. (Tip: Don’t forget to scrape down the sides of the bowl to ensure all ingredients are fully mixed.)
Pour batter into the prepared pan.
Bake at 350°F for 30-32 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Let cake cool in pan for 10 minutes before turning out onto a cooling rack until completely cool.
For the Frosting
Using an electric mixer, cream together cream cheese and whipped topping until well combined. Add powdered sugar and mix until well combined.
Spread frosting evenly on top of the cooled cake. Cover lightly and refrigerate for 2 hours.